1. Field of the Invention
This invention pertains generally to the field of aircraft display units that present flight information to the pilot or flight crew of an aircraft.
2. Description of the Related Art
It is well-known that a Synthetic Vision System (“SVS”) may generate image data representative of a synthetic, three-dimensional perspective of a heading-based scene in front of the aircraft. When provided to a display system, the synthetic image could be presented on the screen of a display unit. Also, it is well-known that an Enhanced Vision System (“EVS”) may generate image data representative of the real-world as an enhanced image of the heading-based scene in front of the aircraft, where such image data has been acquired by one or more fixed or steerable forward-looking image capture devices. When provided to the display system, the enhanced image could be presented on the screen of the display unit. Also, as disclosed by Wenger et al in U.S. Pat. No. 7,605,719, image data from both the SVS and EVS may be combined to form a combined SVS-EVS image that could be presented on the display unit.
Unlike the heading-based synthetic image, a track-based synthetic image (i.e., a synthetic image based on the ground track of the aircraft and not the heading) may have an operational advantage when used in conjunction with Highway-in-the-Sky (“HITS”) symbology. If the aircraft is flying within the HITS, it will be centered within the synthetic image. Also, a Flight Path Vector (“FPV”) symbology may be “clamped” laterally to the center of the synthetic image. The use of the track-based synthetic image could also prevent the unwanted intrusion of the HITS and FPV symbologies into other symbologies (e.g., a speed tape or an altitude tape) when strong crosswinds are present.
The EVS may employ fixed forward-looking image capture devices that are oriented with the heading of the aircraft. When a heading-based SVS image is combined with a heading-based EVS image, there is no misalignment of the images due to crosswinds. When a track-based image is combined with a heading-based image, there is no-misalignment of the images if there is no crosswind; however, in the presence of a crosswind, the alignment between the two images will be lost because a track-based image is referenced to the ground track and the heading-based image is referenced to the heading, As such, there will be a misalignment between the two images resulting in an inconsistent and invalid representation of the track-based scene in front of the aircraft.